Anchor scroll offset

According to **[this][1]** `DOMContentLoaded` is fired when `readyState === 'interactive'` which is not good enough. Using the `load` event might work (since it is supposed to be fired when `readyState === 'complete'`).

For what it's worth, it does work on Firefox, which was the main reason for adding this listener in the first place (as @petebacondarwin exlains https://github.com/angular/angular.js/pull/9517/files#r18675714).

I don't really see how a `$watch` could be placed on `readyState`, since it is updated from outside of the Angular context (so no digest will be triggered).
Am I missing something ?

[1]:  https://developer.mozilla.org/en-US/docs/Web/API/document.readyState